1. Field of the Invention
The present invention relates to a NiH/NiCd (nickel hydrogen/nickel cadmium) battery charger with the function of emergency power supply, and more particularly to a charger in which the batteries can be charged separately in different charging circuits during the charging status. When they are not charged, the batteries in different circuits will be automatically connected in series to supply power as an emergency power supply unit.
2. Description of the Prior Art
Since portable electronic products, such as digital cameras, game players, personal digital assistant (PDA), cellular phone, LED illuminating devices, etc. become more and more popular, a greater capacity of secondary batteries is much required. In addition, the charging time is preferably shorter and the charger's volume is preferably smaller. Therefore, a smaller and cheaper charger with shorter charging time and longer service life is anticipated by the market.
The conventional NiH/NiCd battery chargers have their own advantages and disadvantages. However, they still can't reach the expected effects and need to be improved. These are described as follows:
Referring to FIG. 1, a conventional series charger is shown. This charger utilizes a transformer or an exchange power unit 11 to convert the power mains AC 100˜240V into DC voltage. Meanwhile, the batteries B1, B2, B3, and B4 . . . in series connection are charged by a charging circuit 12. This charging mode has advantages of simple configuration and low price. However, its disadvantage lies in that one is fully charged while the others are not. This will easily cause overcharge of the full-charged battery when the charging process is continued, thereby lowering its service life.
Referring to FIG. 2, a conventional charger in parallel is shown. All parallel-connected batteries B1, B2, B3, and B4 are charged by a charging circuit 12. The advantage thereof lies in that every battery has almost identical charging voltage without disadvantages of overcharge or being not-fully-charged status of the series batteries. However, the greatest disadvantage of the parallel charger lies in that it can't meet the speedy charging requirement. Moreover, the choice and the installation of the electronic control elements are difficult. Supposed that the charging voltage is 2V, and the charging current of each parallel-connected battery is 1.5 A, the required total current will amount to 16 A. However, the diode of the electronic components has problem with internal resistance. In addition, the transistor has problem with pressure difference. To create a low-voltage and high-current charging loop has much to be overcome. For example, it's difficult to fulfill the safety requirement that the wiring of the circuit board has to amount to over 16 mm. Thus, the volume of the parallel charger can't be reduced. Moreover, the components are easily over-heated. Furthermore, the rapid charging requirement can't be fulfilled. If the voltage is increased to enhance the power and to reduce the current value, the four switch transistors Q1, Q2, Q3, Q4 will due to the pressure difference produce a high temperature which is difficult to be overcome. And to add cooling elements will increase the whole volume and the material cost.
The conventional charges, either serial or parallel ones, have disadvantages to some extent. Especially the normal charger is just used now only for charging NiH/NiCd batteries. It is a pity that the other functions of this kind of charger are not used. Therefore, the inventor concentrates on the innovations, in the hope that the current chargers can also supply power as a power supply unit in case of emergency without changing the structure of the conventional charger, increasing the cost, size and using complicated parts. For this purpose, the batteries in the parallel charger must be connected in series to provide sufficient voltage, but it is not very easy. For the four batteries in series, they can provide high voltage, but as above mentioned, there is a disadvantage of overcharging or not full charging which may shorten the battery life. Therefore, the topic how to enlarge the charging capacity as well as to fulfill the function of emergency power supply, will be discussed hereinafter.